Hello Sandeep,
check your Eclipse proxy settings "Windows/Preferences/General/Network Connections" and when you set them correctly continue by reloading all update sites inside your Eclipse. To do so go to "Windows/Preferences/Install Update/Available Software Sites" - go over each one of the listed sites, select them one by one and press "Reload" button.
If the network settings are correct - yore sites will be reloaded and you can proceed with installation.
Anyway - from screenshot I cannot understand what Eclipse version you use, but as this theme is for Juno, just to mention that Juno is no longer supported and you should use Kepler or Luna.
Best regards,
Dobri